1. Field of the Invention
This invention relates to apparatus used to detect pressure waves propagating through a fluid and particularly to a low-noise streamer cable having improved buoyancy compensation.
2. Description of the Related Art
A conventional streamer cable used in seismic exploration may consist of a water-tight jacket enclosing a plurality of pressure sensors disposed at known intervals, depth transducers, and electrical and optical conductors extending through a series of bulkheads. The bulkheads also receive and are anchored to a plurality of steel or nylon stress members extending therethrough. The stress members absorb the strain of towing, thus preventing the cable jacket from stretching and eventually rupturing. The bulkheads provide a cylindrical shape to the streamer cable jacket, separate the stress members from each other, and may be used to protect the enclosed pressure sensors. The streamer cable often is filled with a noncorrosive light kerosene to provide neutral buoyancy while in the water. A few streamer cable designs have employed a closed-cell isocyanurate foam to provide buoyancy and to protect the enclosed sensors from crushing impacts, but closed-cell structure streamer cables are restricted to limited depths of operation.
Conventional streamer cables suffer from substantially high amounts of background noise while under tow. Movement of the streamer cable through the water may result in cable noise on the order of 5 to 10 microvolt. A percentage of this noise may be attributed to sources away from the cable such as the propellers of the ship and the breaking waves in a rough sea. The remainder of the noise is generated by weights and depth controllers attached to the outside of the cable used to attain and maintain a common depth for the length of the cable.
Two mechanisms are generally believed to be responsible for cable-generated noise. In the first mechanism, the cable is subject to irregular towing tension resulting in cable jerk. Shock from cable jerk propagates along the stress members to the bulkheads which in turn generate pressure waves in the cable oil. The pressure waves propagate along the length of the cable section with the cable jacket acting as a wave guide. Rarefractional waves also develop within the cable. The multiplicity of pressure waves and/or rarefractional waves (hereafter called tube waves) are detected by the enclosed pressure sensors as background noise. The second mechanism for generating cable noise may result from objects outside the cable or from the cable moving laterally to the towing direction. That is to say the cable may whip or vibrate; the motion having a range of amplitudes and wave lengths. Cable whip causes the cable jacket to shift about the bulkheads causing vibration to be transferred to the cable oil. Additionally, the cable jacket may impact one or more of the stress members thus generating a pulse which is detected by the sensors. In addition to cable whip, cable noise is generated by vibration of the stress members. Cable jerk pulls the stress members taught from the relaxed position, setting them to vibrate and generate noise.
It is preferred that a streamer cable have substantially zero background noise while in tow, but if this is not readily attainable, it is most desirable to reduce background noise as much as possible.